In front-wheel drive vehicles, as a constant velocity universal joint for transmitting torque from a drive shaft to front wheels at a constant velocity, there has been taken a joint of a tripod type as shown in FIGS. 8A and 8B. In this tripod type, a tripod member 4 has three radially protruding trunnions 11, which carry rollers 13 respectively rotatably fitted through rolling elements 12 over cylindrical outer circumferential surfaces of the trunnions 11, is inserted into an outer member 1 having three cylindrical track grooves 2 formed in an inner surface of and extending axially of the outer member 1. Each of the rollers 13 is accommodated in the corresponding track groove 2 so that it may travel along the track groove 2 rotating about an axis of the trunnion with an outer circumferential surface in engagement with one of a pair of side walls of the track groove 2, which walls provide roller guide surfaces 3 opposing circumferentially of the outer member 1.
In cases where as shown in FIG. 9 the transmission of torque is made in a state that the outer member 1 and the tripod member 4 make a working or operating angle .theta. with respect to each other, each of the rollers 13 and one of the roller guide surfaces 3 of the corresponding track groove 2 come into an obliquely intersecting relation to each other as shown in FIG. 10. In this case, although the roller 13 tends to roll and move in a direction indicated by an arrow a in FIG. 9, because the track groove 2 has a cylindrical configuration parallel to the axial direction of the outer member 1, the roller 13 results in moving while undergoing restriction. Accordingly, the roller 13 slips in relation to the roller guide surface 3 so that the evolution of heat occurs and an axial induced thrust takes place. This induced thrust causes the occurrence of vibration of a vehicle body and the generation of noises, and hence the more reduced induced thrust is desirable.
For a tripod type constant velocity universal joint which is capable of reducing the aforesaid induced thrust to a certain extent, there has been known a two-story construction type comprising a combination of an inner ring and an outer ring as a roller (U.S. Pat. No. 4,786,270). For instance, as shown in FIGS. 11 and 12, in this type of joint, an inner ring 18 is rotatably fitted through rolling elements 12 over a cylindrical outer circumferential surface of a trunnion 11 of a tripod member 4 while an outer ring 19 is rotatably fitted over an outer circumferential surface of the inner ring 18, thus producing a two-story construction. The inner ring 18 has a true-spherical outer circumferential surface 18a with its center on the axis of the trunnion 11, and a cylindrical inner circumferential surface 19a of the outer ring 19 is rotatably fitted over the true-spherical outer circumferential surface 18a. The outer ring 19 is accommodated in the track groove 2 of the outer member 1 so that it may travel in the axial direction of the outer member 1 rolling on a roller guide surface 3 of the track groove 2. In the case that the transmission of torque is made in a state where the outer member 1 and the tripod member 4 make a working angle as shown in FIG. 12, the inner ring 18 is inclined with respect to the outer ring 19, where the inner ring 18 relatively shifts downwardly in FIG. 12 in relation to the cylindrical inner circumferential surface 19a of the outer ring 19. Owing to the occurrence of the relative movement between the inner and outer rings 18, 19, the outer ring 19 is guided along roller guide surfaces 3 of the outer member 1 while maintaining a position parallel to the axial direction of the outer member 1, with the result that it correctly rolls on the roller guide surface 3 with less sliding friction. Thus, the sliding resistance is reducible to suppress the generation of the induced thrust.
In addition, as another tripod type constant velocity universal joint which can reduce, though to a certain extent only, the aforesaid induced thrust, there has been known a type in which a roller is pivotably supported with an outer circumferential surface of a trunnion of a tripod member (U.S. Pat. No. 4,379,706). As shown in FIGS. 13 and 14, this joint features that an outer circumferential surface 22 of a trunnion 21 of a tripod member 4 assumes a curved surface with a gentle curvature. In this case, a cylindrical inner circumferential surface of a roller 24 is pivotably fitted through rolling elements 23 over the outer circumferential surface 22 of the trunnion 21 having the gentle curvature. The roller 24 is accommodated in a track groove 2 of an outer member 1 so that it may travel along the track groove 2. Also in the joint as shown in FIG. 13, when the transmission of torque is made in a state where the outer member 1 and the tripod member 4 make a working angle .theta. as shown in FIG. 14, the cylindrical inner circumferential surface of the roller 24 gets into inclination to the outer circumferential surface 22 of the trunnion 21 and the roller 24 more or less pivots relative to the trunnion 21, which allows the roller 24 to be guided by roller guide surfaces on both sides of the track groove 2 of the outer member 1 to be somewhat in parallel to the axis of the outer member 1. Thus, the roller 24 rolls on the roller guide surface 3 in the axial direction of the outer member 1 so that the sliding resistance of the roller 24 is reducible to lessen the induced thrust.
In the joint with a two-story roller construction as shown in FIG. 11, there has been known the fact that it is possible to stably produce the induced thrust reduction effect when operating in a state with making the working angle. However, this is disadvantageous in that, because the roller is constructed with a combination of an inner ring and an outer ring, in addition to the number of parts for the roller section the number of assembling steps increases to cause the rise of its manufacturing cost. In the case of the FIG. 13 joint, the roller has a simple (one-story) construction as well as the basic structure shown in FIGS. 8A and 8B, so that the rise of the manufacturing cost is avoidable. On the other hand, in this joint, even if the outer member 1 and the tripod member 4 make the working angle .theta. as shown in FIG. 14, it is necessary that, to permit the roller 24 to always maintain the position parallel to the track groove 2, a desirable radial gap g (not shown in the illustration) be defined between the outer circumferential surface 22 of the trunnion 21 and the rolling elements 23. More specifically, as shown in FIG. 15A the radius of curvature (approximate value) in the central portion of the gentle curvature outer circumferential surface 22 of the trunnion 21 is taken to be R and its outer diameter is taken as 2r. When the trunnion 21 is inclined by a working angle .theta. as shown in FIG. 15B, the outer circumferential surface 22 comes into contact with the rolling elements 23 at the portions C, C' where it meets with a chain line b and a chain line c. Since these two portions C, C' stand on the radius [R-(R--r)cos .theta.] larger than the radius r of the central portion, in order to permit the trunnion 21 to smoothly tilt with respect to the roller 24, it is required that as the angle allowing for the tilting therebetween the radial gap g be set to a given value to satisfy g=2{[R-(R-r)cos .theta.]-r}=2(R-r) (1-cos .theta.). In this case, since the radial gap g is proportional to the looseness of the universal joint in its circumferential direction, it is desirable to decrease it to the utmost. Accordingly, in the FIG. 13 joint, difficulty is experienced to maintain the roller 24 to take a position parallel to the track groove 2 at all the required values of working angle .theta. unless the looseness is allowed to increase.